Sunday, August 16, 2015

Is Cohabitation Good for You?

Last week, Ars Technica (and I'm sure other news sites) posted an article on a large-scale survey of health outcomes in Britain, under the headline, "Good news for unmarried couples — cohabitation is good for you" (subtitle: "Married partners tend to be healthy, but living with someone works just as well"). Link.

I'm actually hyper-critical about people who sling around the phrase "correlation does not imply causation" too much in improper cases, but here's a golden example
where it does apply; the headline "cohabitation is good for you", is totally
unwarranted. Now, the findings do say that married & cohabiting
people are healthier than people who live alone. But this could be
either X causes Y, or Y causes X, or other more complicated
interactions. One hypothesis is that "cohabitation is good for you [by
improving health]"; another hypothesis is that "being healthy is good
for your prospects of getting a partner", i.e., healthy people make for
more attractive marriage/cohabitation partners. If you think about it,
I'd say that the latter is actually the more common-sense direction of
the causation here.

How could the direction of this effect be
formally disentangled? Well, you could be on the lookout for a "natural
experiment" where someone who did manage to get married/cohabited breaks
up or gets divorced, and see if their health degrades during the later
period in which they lack a partner. Of course, the researchers here
were smart enough to do exactly that, and an entire paragraph of the Ars
Technica article is in fact devoted to these findings:

"The study
found that changes in status had no obvious impact—the transitions
from/to marriage and nonmarital cohabitation did not have a detrimental
effect on health. There wasn’t an obvious difference in these biomarkers
when participants divorced and then remarried or cohabitated; they
looked the same as participants who remained married. For men who
divorced in their late 30s and didn’t remarry, the risk of metabolic
syndromes in midlife was reduced."

In other words, for anyone in the category of at least being healthy and attractive enough to get married/cohabited once, being married or cohabited made no difference to their health.
Which to my eye is overwhelming evidence that the causation is in the
other direction, i.e., these headlines of "cohabitation is good for you"
are flat-out wrong.